Rigid polyurethane (PUR) foams are made by mixing two components, the "A" component and the "B" component, also known in the art as the "A side" and the "B side". In the manufacture of polyurethane foams, the A side contains the isocyanate and the B side is a mixture of at least one polyol, a catalyst for the formation of urethane linkages, a surfactant and a blowing agent. The catalyst in the B side is gradually deactivated when the formulation is stored for an extended period of time, probably by reaction with small amounts of organic and inorganic acids that are present in the formulation. Producers of foams require that their B side formulations be stable against catalyst deactivation for at least six months at ambient temperature.